Dear (your partner’s name),
I know that you and I have been friends for a long time. I cherish the moments we have shared with each other. I hope that this letter would express what I want to say to you. Pardon me for not saying this directly, but you should know that I really liked you the first time we have met back when we were in high school. I would really be glad if you also feel this way but no matter what your decision is, all I want to say is that I love you and I would like to be always by your side.
We both know that this is not that easy to accept since we have always been close friends. But I do hope that my confession of my love would never ruin our relationship. I love you and I hope that you feel the same way, too.
Lovingly yours,
(Your name)